Many applications for process spectrometers to measure concentrations of measured objects are found in the field of middle infrared radiation (MIR) and, respectively, infrared radiation (IR). These applications are found in the chemical industry, as well as in the fields of food or drink production, or in the pharmaceuticals industry. Pyrodetectors are frequently used for cost effective detection of light in the middle infrared and, respectively, the infrared spectral ranges. These pyrodetectors react to differences in the thermal warming of a sensor element. Therefore, the intensity of the light to be measured is usually modulated in order to obtain an alternating voltage signal at the output of the pyrodetector; the amplitude of the alternating voltage signal is a measure of the absolute intensity to be measured.
Different options are available to obtain such an alternating signal: in the case of mechanical modulation, the beam path is continuously interrupted before the pyrodetector, for example, by using a rotating chopper wheel. This solution leads to significant mechanical complexity and to susceptibilities as regards the mechanical wear of the rotating chopper wheel. Moreover, the dimensions of the spectrometer are enlarged by having to contain the chopper wheel and the pyrodetector.
Another possibility is electrical modulation, in which light from a light source, which transmits a light beam to an optical sensor, is electrically modulated. The light beam received by the optical sensor is changed by a medium to be measured and fed as a measurement signal to the pyrodetector. The electrical modulation also occurs before the light impinges on the pyrodetector in this case. The electrical modulation has the disadvantage that a decreasing modulation depth arises with increasing frequency because of the thermal mass of the light source being used. Moreover, the lifetime of the light source is limited by the temperature cycles, so that a new adjustment of the pyrodetectors must occur in replacing the light source.